Flooding explained: crests, spillways and levees
Townhomes sit in floodwater from the Mississippi River on Monday in Memphis, Tennessee.
May 10th, 2011
09:43 PM ET

Flooding explained: crests, spillways and levees

The swollen Mississippi River on Tuesday was in the process of cresting many feet above flood stage in Memphis, Tennessee, and residents of states to the south are bracing for serious flooding in their communities.

A slow-moving wave of water has been working its way down the river since torrential rains sparked flooding in the Midwest's Ohio and Mississippi river valleys in late April. Hundreds of households were ordered to evacuate in the Memphis area (see pictures), where the river was expected to crest Tuesday around 48 feet - 14 feet above flood stage and less than a foot under the city's record level set in 1937.

The high water is headed for Mississippi and Louisiana, prompting authorities to open one spillway and consider opening another - moves meant to divert some of the water into less populated or unpopulated areas. Up to 5,000 homes will be evacuated in Mississippi, officials there say. In Louisiana, where the river is expected to crest next week, 21 parishes already have issued emergency declarations.

You will hear a lot about crests, spillways and levees over the next couple of weeks. Below you'll find what these mean and what the Army Corps of Engineers and others are doing to mitigate the flooding threat along the Mississippi.

When a river crests

The National Weather Service says a crest is the highest point in a wave. In the case of river flooding, it is the highest stage or level of a flood wave as it passes a particular point. Gauges along the river record the level of the water, and the highest level recorded at each gauge will be the crest for that gauge.

Observers generally know that cresting is occurring when the water level stops rising and becomes stable. However, it's difficult to identify the official crest at the moment that it happens, because during the cresting, small fluctuations happen - water levels can rise a little after they fall a little. The fluctuations are caused by factors such as wind and water currents.

The crest will have passed when the water level continuously decreases. Once this has happened, observers can note what the highest recorded level was. But the cresting of a river at a certain point doesn't mean immediate relief for that area - the water level will lower gradually, meaning flooding can still be a problem days after a crest. On Tuesday, communities in southwestern Illinois and southeastern Missouri still were flooded, days after crests there. In Memphis, although the river was cresting there Tuesday, residents could be dealing with high water levels into June.

Levees meant to keep river in its place

A 2,200-mile system of earthen levees, floodwalls and other controls were built along the Mississippi, Arkansas and Red rivers after a massive flood overwhelmed the southernmost stretch of the Mississippi River in 1927, according to the U.S. Army Corps of Engineers.

The levees guard riverside communities to some extent, but they are rarely tested with the kind of volume and pressure passing through the river now. On Monday in Memphis, the water was moving at 2 million cubic feet per second. At that speed, water would fill a football field at a depth of 44 feet, according to the Army Corps of Engineers.

The system is designed to manage speeds of up to 2,360,000 cubic feet per second at Cairo, Illinois, and up to 3 million cubic feet per second where the Red River meets the Mississippi River in Louisiana. But below the Red River, floodways and spillways (see section below) would be used to divert half that water to other areas in Louisiana so that only up to 1.5 million cubic feet per second flow down the main river channel, according to the Army Corps of Engineers.

Most levees are piles of dirt lined along edges of a river. In theory, vegetation and its roots will keep the dirt stable as water flows by. But levees can fail, in part because moving water can erode, saturate, undermine and destroy them. (Learn more about how levees can fail).

Floodways and spillways

Floodways and spillways are places where the Army Corps of Engineers diverts water from a swollen river. In some cases, this involves intentionally flooding one populated area to prevent or lessen flooding in a more populated area.

On May 2, the Corps intentionally breached a levee near the confluence of the swollen Mississippi and Ohio rivers at the Missouri/Illinois border, sending some of the Mississippi’s water into the New Madrid Floodway - which is 130,000 acres of Missouri farmland.

Missouri officials, wanting to protect the farmers' land, tried unsuccessfully to block the move. The Corps says the move was designed to prevent devastating flooding in Cairo, Illinois, and elsewhere downstream. (See pictures of New Madrid Floodway taken before, after the levee breach.)

The Corps estimates it will take up to two months for the water to recede from the floodway and another month for the land to dry out.

In Norco, Louisiana, the Corps this week opened 72 gates to the Bonnet Carre Spillway, north of New Orleans, diverting millions of gallons of Mississippi River water into Lake Pontchartrain and, eventually, into the Gulf of Mexico.

The agency is considering whether to open the Morganza Spillway, which is on the Mississippi north of Baton Rouge, Louisiana. When the spillway is opened, water would be diverted to the Atchafalaya Delta to the west and south of Baton Rouge, and could flood populated areas such as the town of Morgan City, which has 13,000 residents.

soundoff (141 Responses)
  1. steve

    You guys and ur posts. Get a life, all of you.

    May 15, 2011 at 3:39 pm | Report abuse |
  2. osama joe biden

    plant many trees to avoid floods, most people dont realize or dont care about nature so nature will find ways to get back. TREES!!!

    May 15, 2011 at 4:41 pm | Report abuse |


    May 15, 2011 at 6:20 pm | Report abuse |
    • Ret LAEnvEmp

      Alexandria Louisiana is NOT going to flood from water from the Mississippi River, Morganza Spillway, Atchafalaya River or Basin with flood control work planned by the Army Corps of Engineers. The only real dangers to the major parts of Louisiana are levee total failures upstream of control structures. If a total levee failure washout occurs, a NEW Mississippi River will take off in any direction downhill it wants to go to possibly hundreds of miles from the present river bed/course. In 100 days the New Mississippi River will dump more than enough water to fill Lake Erie in the Great Lakes; this will flood about 1/3 of any State.

      May 16, 2011 at 9:32 am | Report abuse |
  4. content_pessimest

    who would want a house that looks exactily like your neighbors, and whats up with those windows they look fake.

    May 15, 2011 at 6:25 pm | Report abuse |
    • Joe-CT

      Are you trying to make a point here ? I doubt you were trying to imply that because some people choose those type of house to live in that it is not a big deal to flood them, right?
      Making hard choices for the greater good unfortunately has to happen. We need to support those decisions and offer assistance to those who are affected. Thats kind of what decent humans do.

      May 16, 2011 at 11:59 am | Report abuse |
  5. Renea

    I've read all comments to date, & a big "Thank you" to all who have "educated & not irritated" by sharing true knowledge, wisdom & common sense. THANX to Robert & franky for pointing out that there is no area in the U.S. where all Americans can live & be safe from natural disasters. Aside from known threats to any geographical area, are new, unexpected, & EXTREMELY rare natural disasters, such as the 750 year flooding in San Antonio as told by Robert. I live just s.w. of OKC, OK. I've avoided CA in my travels because of the earthquakes, aside from 2 seperate weekend trips to the L.A. area for seminars. In fact, during my 2nd stay there, on approx 9-9-11, our 10th floor hotel room began shaking & shifting, due to a 4.7 earthquake. Unaccustomed to these, we spent several confused seconds looking around at shaking objects, like rattling chest & dresser handles, then at each other. Loud, disrupting & annoying jackhammering in the basement had been ongoing since our arrival, so that was a collective first thought. At about 10 seconds in, I realized what was happening & simply said, "It's an earthquake." Had it been a larger one, we probably would have been hurt, b/c no one acted instantly or INSTINCTIVELY to duck for cover.
    Back to my point: In Oklahoma, we have been experiencing more & stronger earthquakes recently, though I'm not sure when the rise began. We expect, prepare & watch (via weather news, esp during storms) for tornados, but we're not prepared for a large earthquake.

    May 15, 2011 at 7:20 pm | Report abuse |
    • dalewalk

      Thanks for the rant with occassional SCREAMS.
      Now, what was your point??

      May 16, 2011 at 11:08 am | Report abuse |
  6. Renea

    (continued) or most other large-scale disasters, like flooding, major winter storms & so forth. As for moving to avoid tornadoes, my entire family, along with my in-laws, all live here, & I could & would NEVER leave them. Most people feel the same.

    May 15, 2011 at 7:28 pm | Report abuse |
  7. Renea

    Sorry, it was Lars who spoke of the 750 year flood.

    May 15, 2011 at 7:35 pm | Report abuse |
  8. buddyholly137

    I understand that this article was originally put out on May 10th. However, if CNN is going to put it on the front page, it should be edited to reflect current happenings. The article states that "the agency is considering opening up Morganza." From previous articles, Morganza was already opened yesterday. Please proof-read your articles, CNN.

    May 15, 2011 at 7:43 pm | Report abuse |
  9. Art

    I just wish news reporters and news casters would learn to pronounce Atchafalya.

    May 16, 2011 at 6:56 am | Report abuse |
  10. Louis

    Tell me only thing here,I'm not the brightest guy in the world but why didn't they start drawing down the river level a month ago(they knew this water was coming) slowing rather than flooding everything now? If they would have let the level drop down then this would would not have been as bad.

    May 16, 2011 at 9:45 am | Report abuse |
  11. Math snob

    "On Monday in Memphis, the water was moving at 2 million cubic feet per second. At that speed, water would fill a football field at a depth of 44 feet, according to the Army Corps of Engineers."

    This makes no sense unless you say how quickly it will fill the football field. One drop a year will fill it to that depth. It will just take a long time.

    May 16, 2011 at 10:10 am | Report abuse |
    • 4Low

      Math snob, it would fill the football field in ONE SECOND if it is flowing at 2million cubic feet per second, as stated....wake up!

      May 16, 2011 at 11:14 am | Report abuse |
    • Shaun

      I thought the same thing.

      May 16, 2011 at 11:15 am | Report abuse |
  12. truthbtold

    As Gomer Pyle used to say: G-O-O-o-o-o-O-O-O-lly!

    May 16, 2011 at 11:51 am | Report abuse |
  13. David

    What is considered as the football playing field volume. Without the end zone the field is (300' long, 160' wide, 44' deep) = 2,112,000 cubic feet per second. With the end zones its (360' long, 160' wide, 44' deep) = 2,534,400 cubic feet per second. In either case it is alot of water. To bad it was not beer, the fans could have drank it and kept the region dry. Then they could of all run to the Army Corps of Engineers and take a leak for opening the levees.

    May 16, 2011 at 12:12 pm | Report abuse |
  14. cashual

    bull melarky. everyone knows that levees were built to monopolize natural water flows to the wealthy farmers to put the small farmers out of business to buy from them. if the levees weren't built, then a lot of cities or communities wouldnt be built today. if you challenge the order of mother nature...she will prevail always and have her way. levees should have never been built.
    they need to be destroyed and let the natural order have its course as God Intended. illuminati in action by telling us crap like its to keep the waters co.tained in rivers. no human being or group of humans can control nature. lol.

    May 16, 2011 at 12:16 pm | Report abuse |
  15. LINDA

    For the Grace of God it is not us, God give comfort to all that are in harms way and have or will lose all

    May 16, 2011 at 1:29 pm | Report abuse |
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